So you have an existing data plan, and you want to scoop up the next iPhone but you’re afraid that carriers will charge an exorbitant penalty fee that could empty your wallet — well AT&T users are in luck. In light of Apple’s iPhone 5 announcement today, the carrier says that existing data plans would be grandfathered into their iPhone 5 purchase.

“We offer customers the flexibility to keep the iPhone data plans they already have or choose any of our individual or new Mobile Share plans. We’re proud that more customers choose AT&T for iPhone than any other U.S. carrier and look forward to making iPhone 5 the newest addition to our lineup,” Ralph de la Vega, president and CEO of AT&T Mobility, said.

One thing to note is that the iPhone 5 is a 4G LTE smartphone. Existing iPhones use 3G, or for those of you on AT&T and have updated to iOS 5.1, you would have been switched to what AT&T claims to be worthy of the 4G title. Debate over this label hasn’t deterred AT&T from branding its existing service as a 4G network.

“AT&T customers have access to the nation’s largest 4G network, covering 275 million people. Built on top of this widespread 4G network, blazing-fast 4G LTE is available in more than 60 markets today and planned in more than 100 by the end of this year,” the release added.

In reality, Apple would have needed to make some hardware changes, which we’re seeing on the iPhone 5, before AT&T iPhones could claim to be using a true 4G network.

Verizon, after heated backlash for the company’s decision not to grandfather in existing unlimited data plans for users that did not need to switch to a tiered data plan, overturned that decision. Verizon, like AT&T, will grandfather in unlimited plans when users switch to the iPhone, but will not offer to subsidize the cost of an iPhone 5. What that means it that you’re going to have to pay the full retail price of the phone if your two year contract isn’t up by the time the iPhone 5 is stocked on September 21.

AT&T customers can go onto AT&T’s page for the iPhone, and pre-order the iPhone 5 beginning on September 14. And as already announced, the new Apple smartphone will cost $199.99 for the 16GB model, $299.99 for the 32GB model, and $399.99 for the 64GB model, granted that you sign up for a two year data plan.